Substrate binding and catalytic mechanism in ascorbate peroxidase: Evidence for two ascorbate binding sites

The catalytic mechanism of recombinant soybean cytosolic ascorbate peroxidase (rsAPX) and a derivative of rsAPX in which a cysteine residue (Cys32) located close to the substrate (L-ascorbic acid) binding site has been modified to preclude binding of ascorbate [Mandelman, D., Jamal, J., and Poulos, T. L. (1998) Biochemistry 37, 17610-17617] has been examined using pre-steady-state and steady-state kinetic techniques. Formation (k(1) = 3.3 +/- 0.1 X 10(7) M-1 s(-1)) of Compound I and reduction (k(2) = 5.2 +/- 0.3 X 10(6) M-1 s(-1)) of Compound I by substrate are fast. Wavelength maxima for Compound I of rsAPX (lambda(max) (nm) = 409, 530, 569, 655) are consistent with a porphyrin pi-cation radical. Reduction of Compound II by L-ascorbate is rate-limiting: at low substrate concentration (0-500 muM), kinetic traces were monophasic but above similar to500 muM were biphasic. Observed rate constants for the fast phase overlaid with observed rate constants extracted from the (monophasic) dependence observed below 500 muM and showed saturation kinetics; rate constants for the slow phase were linearly dependent on substrate concentration (k(3-slow) = 3.1 +/- 0.1 x 10(3) M-1 s(-1)). Kinetic transients for reduction of Compound 11 by L-ascorbic acid for Cys32-modified rsAPX are monophasic at all substrate concentrations, and the second-order rate constant (k(3) = 0.9 +/- 0.1 X 10(3) M-1 s(-1)) is similar to that obtained from the slow phase of Compound 11 reduction for unmodified rsAPX. Steady-state oxidation Of L-ascorbate by rsAPX showed a sigmoidal dependence on substrate concentration and data were satisfactorily rationalized using the Hill equation; oxidation Of L-ascorbic acid by Cys32-modified rsAPX showed no evidence of sigmoidal behavior. The data are consistent with the presence of two kinetically competent binding sites for ascorbate in APX.